In Fe-Si alloy and Fe-Si-Al alloy, there are Fe-6.5%Si alloy and Fe-9.6%Si-5.4%Al alloy (sendust) which have very high magnetic permeability and excellent soft magnetic characteristics. Especially, the sendust has been applied to electronic instrumentalities such as dust cores, magnetic heads and others since its invention in 1973. With respect to the magnetic head, a high coercive field strength of recording media has advanced nowadays, accompanying high density of magnetic recording media, and the sendust of high saturated magnetization has been interested, since this material is more suitable to the recoding than ferrite head used conventionally. Since Fe-6.5%Si alloy has high saturation flux density, this material is considered to be applied to iron cores of transformers, or other electric, and electronic instrumentalities.
A problem when these high Si alloys which are excellent in the soft magnetic characteristics are used for electronic parts, is that they could not be rolled in thin shape, since they are brittle. Therefore, the sendust is sliced after forging to produce thin pieces for the magnetic heads, which is, however, a process very inferior in efficiency in the production of the heads. Besides, the sendust is easily formed with cracks or pinholes during solidification after casting, and those defects should be removed for which, however, a further process is required.
For solving the problems involved with the above mentioned manufacturing process, the under mentioned processes have been proposed.
(1) Rolling and deforming in hot work
(2) Improvement of workability by addition of elements
(3) Direct production by rapid solidification
(4) Composition control after rolling
The above mentioned process (1) is made possible by super slow strain rate at the temperature of more than 1000.degree. C., however it would invite much difficulties in practising such a condition industrially. The attempt (2) more or less improves the workability by adding the elements, but the material is brittle, and an application to the thin sheet is difficult and the added elements deteriorates the magnetism. The process (3) directly casts the molten metal into the thin shape, and is very useful to the brittle material in regard to production of the thin sheets without the rolling process. The control (4) comprises, melting low Si or low Al steel, rolling it in thin shape, enriching Si or Al by penetration from the surface thereof, and finally producing high Si thin steel sheets.
However, since conventionally proposed penetrating processes take penetration treating time as long as more than 30 minutes and temperatures as high as about 1230.degree. C., the shapes after the penetrating treatment are undesirable. Further, the most fatal phenomenon in the prior art to the production of the high magnetic permeable materials is generation of large voids called Kirkendall void which accompany the penetration. These voids remain in spite of the sintering treatment, so that the magnetic permeability is considerably declined. The reason why a process of producing high Si thin steel sheet by the Si penetration has not yet been realized, is that it is difficult to remove the voids.